Do you mean twist like move or twist like twisted metal?
Guys I'm getting confused with the terminology used for rear drop outs. Vertical dropouts allow the rear axle to be removed like the front fork, pretty much straight down. It's the axle style used for 21 speed etc. bikes with derail. drive trains for which it's wonderful, but it's lousy for single speed motor powered bikes p.m. me if you want to read the book of why this is.
The horizontal rear axle drop slot is parallel to the ground and the axle slips out by pulling straight to the rear of the frame. This allows easy wheel removal and allows two chain adjustment by simply altering the rear axle position. The slot for this is cut horizontally relative to the ground and the vertical drop is at 90 degrees relative to the ground. Chain adjustment can't be made by moving the axle in the vertical drop design as the axle is always locked into the drop slot.
Terms now defined: it's literally impossible for the rear axle to slip off on the vertical slot drop out. The axle could fall out if the bike got airborne and the axle was very loose, but it can't slip off unless the drop plate fractures and separates from the frame.... one in a billion chance that happens.
The horizontal slot that is preferred for motor bike construction, can come loose if not properly tightened, but as for the wheel falling off, not going to happen: if the chains are on and properly adjusted they will hold the wheel in the slot, and in addition if you are using a coaster brake the brake lever frame band will keep the wheel on. If you're not riding under the influence you should notice something is wrong back there long before damage occurs, the first major event would be chains breaking or one or both chains jumping sprocket. You should feel the bike getting wormy long before these things occur though.
Components located on the seat post that deal with drive train can't be allowed slip, twist or break. If you're not an accomplished welder hire one, some bolt on stuff is fine but typically pricy as well. Study forum builds that are designed like what you want to use and learn stuff, ask these builders what they used etc. Private message is usually the best way to get a response from forum members.
Have fun,
Rick C.